You were right, it should be immunostained. The term is actually immunostaining.

"is a general term in biochemistry that applies to any use of an antibody-based method to detect a specific protein in a sample. The term immunostaining was originally used to refer to the immunohistochemical staining of tissue sections, as first described by Albert Coons in 1941.[1] Now however, immunostaining encompasses a broad range of techniques used in histology, cell biology, and molecular biology that utilise antibody-based staining methods."

Definitely take a look at the Dutch site, because you can find a lot of defintions and Dutch terms.